Effects of methanol blending on combustion characteristics and various emissions of a diesel engine fueled with soybean biodiesel

•Methanol addition has opposite effect on peak HRR under low and high loads.•Methanol/biodiesel blends reduced the 1,3-Butadiene and benzene emissions.•The addition of methanol has no obvious benefit in reducing regulated emissions. A 4-cylinder direct-injection diesel engine was tested using pure soybean biodiesel, 10% or 20% methanol-biodiesel mixture as fuels. Loads were set at 0.13, 0.38, 0.63, 0.88 and 1.13 MPa BMEP, while the engine speed was 1800 rpm. Other related parameters including the main injection angle were unchanged. The cylinder pressure, heat release rate, ignition delay, combustion duration, maximum pressure rise rate, regulated emissions and unregulated emissions were comparatively analyzed among these three fuels. The impacts of methanol blending on the combustion and emission behaviors of biodiesel under different loads were investigated. The methanol mixing affected the peak cylinder pressure and peak heat release rate in completely opposite ways between low load and medium-to-high load. The ignition delay and combustion duration may be extended at large blending ratio and low load, but were shortened in other cases by the blending with methanol. The effects of methanol mixing on the maximum pressure rise rate were fully opposite between low load and medium-to-high load and delayed the appearance of the curve peak. At 0.13 MPa BMEP, the presence of methanol significantly promoted the emissions of CO and 1,3-butadiene, and decreased the emissions under other loads. With the rise of methanol mixing ratio, the emissions of HC and acetaldehyde were raised, while less benzene was emitted under all loads. At 0.13 MPa BMEP, adding sufficient methanol reduced the NOX emissions, while under other loads, more NOX was emitted with the rise in methanol addition.